This application claims the benefit of Korean Application No. P00-29197, filed in the Republic of Korea on May 30, 2000, which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display having a reflective type display.
2. Description of the Related Art
A liquid crystal display (LCD) has many advantages. One advantage is that the LCD has a flat panel display. The flat display is advantageous because it is thin and not bulky. The LCD also has low power consumption. The LCD is used in many applications, for example, portable computers such as personal computer notebooks, office automation and audio-visual equipment. The LCD displays a picture or image when it manipulates an electric field that is applied to a liquid crystal material having a dielectric anisotropy to transmit or shut off a light. The LCD emits an external light rather than having a light generated from within, which is different from display devices such as electro-luminescence (EL) devices, cathode ray tubes (CRT), light emitting diodes (LED), and similar devices.
The LCD is largely classified into either a transmissive or reflective type display, depending on the manner in which the light is emitted. The transmissive LCD includes a liquid crystal panel having liquid crystal material injected between two glass substrates, and a back light for supplying a light to the liquid crystal panel. However, it is difficult to make a transmissive LCD, which is thin and light weight because of the added bulk and weight of the back light. Another drawback is that the back light causes excessive power consumption.
The reflective LCD has been widely used as a portable display device such as in an electronic passport or a personal data association (PDA) because it does not require a back light with low power consumption. The reflective LCD having less than sixteen scanning lines usually includes a twist nematic liquid crystal mode that has a 90xc2x0 twist angle. The reflective LCD having more than sixteen scanning lines, however, usually includes a super twist nematic liquid crystal mode that engages a Diechroic polarizer or a phase compensating plate. The reflective LCDs that are available in the market have adopted a scheme where the device emits the difference between an electro-optical transmission curve of red, green, and blue by allowing xcex94nxe2x80xa2d of the super twist nematics to be greater than 1.0 xcexcm, or in the alternative, attaching color filters. A polarization-modulated type LCD that uses a polarizing plate and a reflecting plate that depends on a very large viewing angle, whereas a reflective LCD that uses an active matrix can realize various colors.
Referring to FIG. 1, the conventional reflective LCD includes a polarizer 2 for polarizing a natural light into a linear polarized light. The linear polarized light then transmits through a retardation film 4, which converts the linear polarized light into a circular polarized light. A glass substrate 6 transmits the circular polarized light to a color filter 8, which is arranged in red, green, and blue pixels. A liquid crystal layer 12 then converts the circular polarized light into a linear polarized light. A reflective plate 14 reflects light that passes through the liquid crystal layer 12.
As shown in FIG. 2A, when a voltage is not applied to the conventional reflective LCD, only a first linear polarized light 31 (e.g., S wave) found in an incident light 30 that is mingled in a natural light and a peripheral light transmits through the polarizer 2. The first linear polarized light 31 having transmitted through the polarizer 2 is then converted into a right-handed circularly polarized light 32 by means of a retardation film 4 having a phase difference value of xcex/4. The right-handed circularly polarized light 32 transmits through the glass substrate 6 as it is. After the right-handed circularly polarized light 32 transmits through the glass substrate 6, it transmits through a red(R), green(G), or blue(B) color filter 8 of an absorptive color filter, thereby having a specific wavelength. The right-handed circularly polarized light 32 transmits through a liquid crystal layer 12 after having transmitted through the color filter 8.
The liquid crystal layer 12 having a phase difference value of xcex/4 is injected into the liquid crystal display panel. It is changed into a second linear polarized light 33 (e.g., P wave) that is perpendicular to the first linear polarized light 31. The light changed into the second linear polarized light 33 is again forward-reflected by the reflective plate to be irradiated onto the liquid crystal layer 12. The irradiated second linear polarized light 33a is converted into a right-handed circularly polarized light 32a by transmitting through the liquid crystal layer 12. The right-handed circularly polarized light 32a then transmits through the absorptive color filter 8 again. After the right-handed circularly polarized light 32a has transmitted through the absorptive color filter 8, it is converted again into a first linear polarized light 31a by means of the retardation film 4. This first linear polarized light 31a transmits through the polarizer 2 to display a specific color on a screen (not shown) of the reflective LCD.
As shown in FIG. 2B, when a voltage is applied to the conventional reflective LCD, only a first linear polarized light found in an incident light 30 that is mingled in a natural light and a peripheral light transmits through the polarizer 2. The first linear polarized light 31 having transmitted through the polarizer 2 is then converted into a right-handed circularly polarized light 32 by means of a retardation film 4 having a phase difference value of xcex/4. The right-handed circularly polarized light 32 then transmits through the glass substrate 6. After the right-handed circularly polarized light 32 transmits through the glass substrate 6, it transmits through a red(R), green(G), or blue(B) color filter 8 of an absorptive color filter. The right-handed circularly polarized light 32 is then irradiated onto the liquid crystal layer 12 in the state of a right-handed circularly polarized light having a specific wavelength. Since the liquid crystal layer 12 is supplied with a voltage current, the right-handed circularly polarized light 33 irradiated onto the liquid crystal layer 12 is also irradiated onto a reflective plate 14 in the same state, without any change.
The right-handed circularly polarized light 33 irradiated onto the reflective plate 14 is then converted into a left-handed circularly polarized light 33a having a phase change of 180xc2x0 and reflected. The reflected left-handed circularly polarized light 33a is then irradiated onto the liquid crystal, which is injected into the liquid crystal panel. When the liquid crystal layer 12 is supplied with a voltage current, the left-handed circularly polarized light 32a irradiated onto the liquid crystal layer 12 then transmits through the absorptive color filter 8 in the same state, without any change. The left-handed circularly polarized light 32a transmitted through the absorptive color filter 8 is then converted into a second linear polarized light 31a by means of the retardation film 4. Light that is converted into the second linear-polarized light 31a fails to transmit through the polarizer 2 because the polarizer 2 is only capable of transmitting a first linear polarized light 31, thus, the screen of the reflective LCD is only allowed to be in a blackened state.
Conventional reflective LCDs are disadvantageous in that the absorptive color filter is positioned on the upper substrate. With the absorptive color filter positioned on the upper substrate, a natural or peripheral light must transmit through the color filter twice between the time it enters and exits the device. For this reason, the reflection efficiency of a light transmitting through the LCD panel is low. Even if the light efficiency is increased in conventional reflective LCDs by using color filters, the color purity is deteriorated because color filters that have poor purity must be used.
In view of the above problems a reflective liquid crystal display according to one aspect of the present invention can include an array substrate for a reflective liquid crystal display including a substrate, thin film elements formed on the substrate, and color filters formed on the thin film elements. Each of the color filters preferably includes a cholesteric liquid crystal. A pixel electrode can be formed on each of the color filters, wherein the pixel electrode corresponds to a respective color filter.
In another aspect, the reflective liquid crystal display device can include a first substrate, a retardation film disposed on the first substrate, and a polarizing plate disposed on the retardation film. A transparent electrode can be formed on the lower side of the first substrate. A lower plate preferably includes a second substrate, thin film elements formed on the second substrate, and color filters formed on the thin film elements. Each of the color filters preferably includes a cholestric liquid crystal. A pixel electrode can be formed on each of the color filters, wherein the pixel corresponds to a respective color filter. A liquid crystal layer can be filled between the first substrate and the second substrate.